Novel nanophotonic chip improves the bandwidth of twisted light communications

As a green technology, light has played a pivotal role in the age of information technologies. The ever-increasing demand for high-capacity telecommunications has propelled the use of physically orthogonal dimensions of light (e.g., brightness, color, polarization, and mode) for information multiplexing (a process in which parallel light channels are used in a shared medium to produce an increased capacity). The angular momentum (AM) of light includes spin angular momentum (SAM)—possessed by circularly polarized light—and orbital angular momentum (OAM), which is exhibited by the twisted wavefront of light. AM has thus emerged as a new multiplexing method for high-capacity communications, such as free-space¹ and compact optical fiber² approaches. The realization of on-chip AM multiplexing, however, remains a challenge because there is no natural material that can be used to sense twisted light.

In conventional AM multiplexing approaches, macroscale interference-based detection methods—such as hologram coding^{1, 2} or phase shifting³—of OAM-carrying beams have been used exclusively. Unfortunately, these methods impose a fundamental physical limit on realizing AM multiplexing at chip-scale footprint resolutions. Although surface plasmon polaritons (SPPs), which are capable of strong light confinement, have long been pursued for overcoming the size limitation of nanophotonic devices, the interference-based OAM sensitivity of a holographic metasurface⁴ inevitably degrades the perceptive devices for on-chip applications.

In our recent work we have therefore designed an integrated nanophotonic chip with which we can achieve non-interference AM sensitivity, with the use of mode-sorting nanoring slits.⁵ With our device we offer unparalleled levels of control over the AM of light, which paves the way for ultrawide bandwidth telecommunications. The principle of our on-chip non-interference AM sensitivity is illustrated in Figure 1. For our devices, we engraved nanogroove structures onto a metallic film. This allows AM modes carried by photons to be converted into SPPs.

Figure 1. Schematic diagram of a spiral nanogroove engraved in a metallic film, which is used to achieve on-chip non-interference angular momentum (AM) sensitivity. The inset shows the enclosed nanoring slit with a fixed width (w) and inner radius (R_in). SPP: Surface plasmon polariton.

We show an example of a spiral nanogroove coupler in Figure 1. We calculated the converted plasmonic AM mode for this coupler from the total AM mode (L), i.e., L=s+l₀+l_s (where s and l₀ are the modal indices for SAM and OAM, respectively, and where l_s is the geometrical topological charge that arises from the spiral nanogroove). A nanoring slit, with a fixed width of 50nm, is enclosed in the geometrical center of the nanogroove, as shown in the inset to Figure 1. This nanoring slit offers a distinctive outcoupling efficiency on the plasmonic AM nodes, and we therefore achieved non-interference AM sensitivity at the nanoscale.

To investigate the non-interference AM sensitivity readability from the mode-sorting nanoring slits, we defined a mode-matching factor in which the eigen-AM mode (supported by the nanoring slits) and the plasmonic AM mode (excited from the nanogrooves) are matched.⁵ With this factor, we reveal that plasmonic modes with L of ±1 and ±3 can be distinctively coupled-out with the use of nanoring slits that have an inner radius (R_in) of 200 and 500nm, respectively.⁵ Moreover, we discovered that the non-interference AM sensitivity is naturally non-resonant. This leads to an ultrabroad band at telecommunication wavelengths (i.e., 1.45–1.65μm).⁵

We illustrate the concept of on-chip AM multiplexing for ultrawide bandwidth telecommunications in Figure 2. Without losing the generality, AM-carrying beams with four selection AM modes (i.e., l₀=−2, s=−1; l₀=−1, s=−1; l₀=+1, s=+1; and l₀=+2, s=+1) propagate through a nanoring aperture (NRA) multiplexing unit. This unit—see Figure 2(a)—consists of two spiral nanogrooves, with l_s of +1 (left) and –1 (right), as well as different-sized spatially shifted nanoring slits. By using the AM-carrying beams to illuminate the NRA, we are able to route the coupled plasmonic AM modes to the locations of the nanoring slits. The AM signals are then distinctively transmitted through the slits, without loss of their carried information, as illustrated in Figure 2(b).

Figure 2. Illustrating the on-chip AM multiplexing for ultrawide bandwidth telecommunications concept. (a) Schematic diagram of a nanoring aperture (NRA) multiplexing unit that consists of two spiral nanogrooves, with geometrical topological charges (l_s) of +1 (left) and –1 (right), and two concentric double nanoring slits (with different sizes and spatial locations). (b) Far-field intensity patterns of four AM-carrying beams that are transmitted from the NRA. s and l₀: Modal indices for spin angular momentum and orbital angular momentum, respectively. (c) An NRA-structured AM-multiplexing chip can be used to achieve ultrawide bandwidth telecommunications through parallel processing of an AM-carrying fiber bundle.

In our work we have also shown that a large-scale NRA-structured AM-multiplexing chip (NAMMC) can be used for on-chip processing of AM information in parallel, through a multibeam approach.⁵ Our NAMMC consists of an array of individually addressable NRAs, i.e., where the NRA units are separated by a spacing that is larger than the diffraction-limit distance. With our nanophotonic NAMMC, we thus lay the foundation for ultrawide bandwidth telecommunications. Furthermore, because of the rapid development of nanofabrication technology, there is no technical challenge to the mass production of such chips. Our method can therefore potentially boost the bandwidth of optical technologies that are based on current cable technology, i.e., where a large amount of information in one fiber bundle—see Figure 2(c)—can be processed in parallel through the NAMMC.

In summary, we have demonstrated a new nanophotonic chip, with engraved spiral nanogrooves and enclosed nanoring slits, which can be used to achieve non-interference angular momentum sensitivity. Our research represents a major technology breakthrough, and opens up a new perspective on employing light for on-chip information generation and transmission, as well as the retrieval of images, videos, and sounds. Our work also paves the way for the next generation of compact and ultrawide bandwidth telecommunications, lays the foundation for the future ultrabroadband big data industry, and provides a new platform for the next industry revolution. In the next stage of our work, we plan to attach the NAMMC to an AM-carrying fiber bundle, to realize parallel on-chip AM processing with a dramatically improved bandwidth.